Free-radical cure of ethylene-propylene copolymer rubber with supported nitrogen oxide as a promoter



United States Patent 3,157,625 FREE-RADECAL C 1 13 QB ETHYLENE-PRQPYL- ENE COPOLYM'ER RUEEER WETH SUTPQRTED NTI'RQGEN QXEDE AS A PROMQTER Edwin T. Yates, Highland hark, and Harold K. Garner,

Wayne, Ni, aesignors to United States Rubber Cornpmy, New York, N.Y., a corporation of New Eersey No Drawing. Filed .l'uiy 12, 1962, Ser. No. 209,313 3 Claims. (Cl. 2643-832) This invention relates to a method of curing ethylene/ propylene elastomers, and more particularly it relates to a method of assisting the free-radical cure of rubbery copolymers of ethylene and propylene, by a nitrogen oxide.

It has been known for some time that numerous materials improve the vulcanization of rubbery high polymers by free-radical sources. Robinson et al., in Canadian Patent 599,198, May 31, 1960, describe the improvement in the physical properties of rubbery propylene copolyrner vulcanizates cured by the joint action of a free-radical source, such as dicumyl peroxide, and a dinitroso compound, such as N-methyl-N,4-dinitrosoaniline.

Our invention is based on the discovery that the use of nitric oxide or nitrogen dioxide in conjunction with an organic peroxide produces an excellent cure of saturated ethylene/propylene copolymer rubbers. The fact that these oxides of nitrogen do aid in the free-radical crosslinking of these saturated hydrocarbon rubbers is surprising since the prior art appears to emphasize the importance of there being an organic moiety in the nitroso compounds. Also, Kharasch et al., Ind. Eng. Chem. 39, 830 (1947), suggest that nitrogen dioxide destroys peroxides. Therefore, it was most unexpected to find that the oxides of nitrogen would not inhibit the cross-linking reaction, but enhance it.

In the practice of our invention We prefer to use the oxide of nitrogen adsorbed or absorbed on or in a solid, finely divided support. The supports used may include, by way of examples, aluminosilicates in the form of molecular sieves, carbon black fillers commonly used in rubber compounding, and rubber-grade silica. Hersh, Molecular Sieves, Reinhold, New York, 196 1, at the preface'and at page 3, describes Linde molecular sieves as synthetic, crystalline metal aluminosilicates. Hi-Sil is an example of a very fine particle size hydrated silica, described in Bulletin No. 4 of Columbia Southern Chemical Corporation, dated April 1960.

Examples of the molecular sieves which may be used in the invention are described in the following references:

(1) Sequestration of Molecules With Aid of Crystal Sieves, by R. M. Banner, in Brennstoff-Chem, 35, 325 (1954). v

(2) Zeolites as Absorbents and Molecular Sieves, by R. M. Banner, Ann. Reports, 41, 31 (1944). V

(3) Inclusion Complexes of Faujasite With Parafiins and Permanent Gases, by Banner and Sutherland, Proc. Roy. Soc., 237, 439 (1956).

The rubbers may containthe usual type of rubber fillers such as carbon blacks, silicates, etc., and age-improvers, oils and other common ingredients.

To impregnate the support with the oxide of nitrogen,

' a weighted amount of the support is placed in a-vacuum desiccator, the pressure is reduced by use of a vacuum pump, then the oxide of nitrogen is bled in until the pres- Patented Nov. 17, 1964 "ice Percent Oxide of Nitrogen by weight (based on the weight of the support plus s) Promoter Identi fication Oxide of Nitrogen Support I. II. III. IV.

*Faujasite which has the composition expressed in terms of oxides as NazOz:Al2O3:2.67SiO2:2H O and which has a free diameter in channels of about 12 angstroms.

added to the rubber, along with the free-radical curative and filler, on a standard two-roll mill or in a Banbury type mixer, and the cures are made in conventional curing presses.

The rubbery ethylene/propylene copolymers actually used had intrinsic viscosities, in cyclohexane at 30 C., of from 4.0 to 6.9. The propylene content varied from to percent by weight. However, any rubbery copolymer of ethylene/propylene is satisfactory, including those containing from 40 to propylene.

The temperaure for the cross-linking is that which is required to generate the free-radical from the chosen source at a useful rate. The half-life for a number of peroxides is given by Doenhnert and Mageli, Modern Plastics, 36, [6] 142 (1959) and is helpful in choosing a suitable temperature range for members of this class of free-radical sources. Generally, the peroxide appropriately has a half-life of at least five minutes at 275 F., but less than one hour at 375 F.

Sources of free-radicals are well known in the art and are described, for example, in Canadian Patent'599,l98 referred to above.

Thus, as free-radical generators there may be used organic or inorganic compounds which break down under the influence of heat, friction, irradiation with ultraviolet, X-rays, or rays emanating from radioactive materials. Free-radicals may also be produced directly from the materials to be coupled by the action of oxidizing agents, ul-

traviolet, X-rays, or gamma radiation emanating from a metrical or bis(aralkyl) and bis(alkyl) peroxides: di-

benzyl peroxide, bis(alpha-methylbenzyl') peroxide, bis

(alpha,alpha-'dimethylnaphthylmethyl) peroxide, bis(alfactory.

more detail.

pha,alpha-dimethyl-p-methylbenzyl) peroxide, bis(alpha, V

alpha-dimethyl-p-isopropylbenzyl) peroxide, and di-tertbutyl peroxide.

Unsymmetrical peroxides useful in the invention'include the. following compounds: benzyl (alpha-methyl- I v persulfate, or alkali or alkaline earth peroxides (e.g. so-

dium peroxide) on heating form initiating free-radicals in the process of this invention. Other free-radical formers are the azo compounds such as alpha, alpha-azobis (isobutyronitrile) The cross-linking temperature depends on the source of. free-radicals since elevated temperatures are not re quired for cross-linking and are used only when required for the creation of free-radicals from the. free-radical generator, or when the polymer is subjected to molding conditions during the cross-linking process. When heat is used to develop free-radicals frornidiaralkyl peroxides a temperature in the range of 270-4807 F. is preferred. Whenmeans other than heat is used to. develop the freeradicals in 'l0 F. may be used. The temperature during crosslinking may thus be anywhere in the range of about l F. to about 480 F. and the temperature is selected according to the need with respect to free-radical generatiori', molding or the like.

We strongly prefer the organic peroxides, especially the di-tert-allryl peroxides, which term includes dicumyl peroxide, since these are readily available and are the most practical.

V The free-radical source is used in the amount required,"

in combination with the promoter of the invention,-i.e., the nitrogen oxide, to impmt the desired physical properties to the rubber. Generally, these will'be'used in'relatively minor amounts compared to the weight of the rubber. From about .5 to 20 parts of peroxide per 100 of rubber can be used. However, one will usually use from about 1 to about 1.0 parts per 100 of rubber. The amount of the oxide of nitrogen to be used depends also upon the degree or" cure desired and is not critical. About .05 to 10 parts byweight per 100 of the rubber is satis- Usually, an amount in the lower part of this range'is adequate, e.g., fromabout .1 to about 5'parts per 1000f rubber.

Satisfactory results are usually obtained by heating'the mixtureat a temperature of from about 265 F. to about 480 F. for a period of time ranging from about one minute to about five hours. Usually, the timeand' temperature will be inversely related, and the exact combination of time and temperature of cure will depend upon such variables as the extent of cure'desired, the si'ze of the article, the concentration of curing agents,'the particular curing agents employed, the character of the heating device, etc.

the system, temperatures as low as about The following examples illustrate the inventionin 7 Example I on a silica support as a promoter(identifiedabove as pared by placing a weighed quantitylof silica (in an open Tnislexample illustrates the use of nitrogen dioxide dish), in a vacuum desiccator; evacuating the desiccator v V andthen bleeding in nitrogen dioxide until the pressure 1s.;about 5 centimeters'less than the prevailing atmos phericpress'ure, as describedtpreviously.; Thesilica is carbon black for advancing the state of cure of ethylene/ propylene elastomer by the free-radical source, dicumyl peroxide. L I Parts by Weight .Materlals V Yr 1 V Iii E Mix F I (Control) (Invention) V Ethylene/propylene (7 propylene) V .elastomer. 0 i 100, Carbonblac 56.5 50. V -Dicumylpero 7' e 7 PromoterIII (described above); 6, 6

1 OureTirrieat320R j I (min) 15 gas e0 15 30 so 7 Properties: 7 v a a V i I Tensile,p.s.i 720 1, 035 1,125 1,565 1,820 1,8oa'

Elongation at break, 1.. 3

percent". 555' i 4% 495 575 460 V 460.." Modulus .at "300% 7 p.s.i .42 0 j 560 7 (360 910 Y 935" W t. of gas absorbed-kn t. of silica Parts by Weight 7 Materials 7 7 Mix A Mix B (Control) (Invention) Ethylene/propylene copolymer' rubber (67% propylene) 100 100 Carbon black 50 50 Dicurnyl peroxide (40% active) 7 7 romoter I (described above). 2. 8 Hi-Sil 2.6 Cure Time at 320 F.

(min.). 15 30 V 60 15 30 (it) Properties: 7 a i Tensile, p.s.i 975 1, 055 995 2,150' 2, 216 2,140 Elongation at break,

perccntflm; 595 500 585 585 515 525 Modulus at 300%,

psi 330 00 370 725 810 755 The data showthat the presence of theNO greatly enhances the state of cure; e

. Example II I This example demonstrates the use of nitric oxide on molecular sieves for promoting the cure of ethylene/propylene rubbers by a source of tree-radicals.

Parts by W'eight Materials H Mix 0 Mix D (Control) (Invention) Ethylene/propylene (70% propylene) rubber 100 100 Carbonblaek. 50 50 Dicumylperoxide (40% active) 7 7 Promoter 11 (described above)" 11.8 Linde 13% molecular sieves 11.4

Cure Time at 320 F. Y (min) 15 30 60' 15 30' 60 Properties:

Tensile, .s.i- 820 1, 090 1,100 1, 965 1, 840 2, 075 Elongation at bre percent 545 505 500 470 ilO 450 Modulus at 300%, i

p.s.i 420 530 545 910 1, 075 1 050 It can be seen that stock D, containing the nitric oxide, has reached a much more useful state of cure thanhas control stock 1C. V. w V a Example 111 V Thisexarnpledemonstrates the use of nitric oxideon Higher arnounts of promoter 131 than used in mix F give a still further increase in tensile and modulus, With only a moderate reduction in elongation.

Example IV This example demonstrates the use of nitrogen dioxide impregnated in coconut charcoal for promoting the treeradical cross-linking of ethylene/ propylene rubber.

Parts by Weight Materials 11in G lvfili H (Control) (Invention) Ethylene/propylene (67% propylene) rubber 100 100 Carbon black 50 50 Dicumyl peroxide (40% active) l 7 7 Promoter IV (described ahove) 0. 5 Coconut charcoal 0.3

Cure Time at 320 F.

Properties:

Tensile,p.s.i 1,315 1,530 1,500 1,930 1,750 1,840 Elongation at break,

percent 510 500 400 470 390 415 Modulus at 300%,

p.s.i 535 635 625 815 1, 040 1,000

It is seen that stock H, which contains nitrogen dioxide, has reached a much more useful state of cure than has control stock G, for the same cure times.

The method of the invention is useful in making fabricated or shaped articles of all sorts, partly or entirely irom ethylene/ propylene rubber. Thus, ethylene/propylene rubber compounded for cure with a free-radical source and a nitrogen oxide in accordance with the invention may be used to fabricate pneumatic tires or parts thereof (eg. tread, carcass, sidewall) in the conventional manner, followed by heating to efiect the cure. Shoes, hose, clothing, belts, floor coverings, molded goods and mechanical goods or" all sorts, with or without reinforcement such as textile or wire reinforcement, may be made from compositions compounded for cure in accordance with the invention. Coated fabrics of particular utility may be made in this manner. Blown or expanded articles may be made by including a blowing agent in the composition. The composition may be shaped by extrusion, calendering, molding, or any other suitable method. The composition may be blended with other materials, such as resins or rubbers (e.g., butyl rubber).

Having thus described our invention, what we claim and desire to protect by letters Patent is:

1. In a method of curing ethylene/propylene copolymer rubber by heating said copolymer rubber in the admixture with a free-radical source and a promoter, the improvement comprising employing as the said promoter a nitrogen oxide selected from the group consisting of nitric oxide and nitrogen dioxide, adsorbed on a solid, finely divided support.

2. A method as in claim 1, in which the said freeradical source is an organic peroxide.

3. A method as in claim 1, in which the said fleeradical source is dicurnyl peroxide.

References (Iited in the file of this patent UNITED STATES PATENTS 3,088,930 Cain et al May 7, 1963 3,093,614 Muckenzie June 11, 1963' FOREIGN PATENTS 886,897 Great Britain Ian. 10, 1962 

1. IN A METHOD OF CURING ETHYLENE/PROPYLENE COPOLYMER RUBBER BY HEATING SAID COPOLYMER RUBBER IN THE ADMIXTURE WITH A FREE-RADICAL SOURCE AND A PROMOTER, THE IMPROVEMENT COMPRISING EMPLOYING AS THE SAID PROMOTER A NITROGEN OXIDE SELECTED FROM THE GROUP CONSISTING OF NITRIC OXIDE AND NITROGEN DIOXIDE, ADSROBED ON A SOLID, FINELY DIVIDED SUPPORT. 